1. Field of the Invention
The present invention relates to an image forming apparatus employing inkjet printing.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multifunction apparatus combining several of the capabilities of the above devices, for example, an inkjet recording apparatus including an ink droplet discharge head to impact ink droplets and form images on a medium while conveying the medium by adhering the ink droplets to, for example, a sheet, is known.
Such an image forming apparatus employing the droplet discharge recording method includes a serial-type inkjet printer, which performs printing while scanning the print head by a carriage unit in a direction perpendicular to a sheet conveyance direction, i.e., in a main scanning direction. For a serial-type inkjet printer to perform printing properly, a device to detect a position of the carriage and its extent of movement is necessary. For example, JP-2006-187905A discloses a method using an elongated linear encoder scale in the carriage shifting direction.
FIG. 9 shows a carriage 1 of a known inkjet printer with a linear encoder scale 7. The linear encoder scale 7 generally includes scale marks at an interval of 150 dpi or more. An encoder sensor 8 disposed on the carriage 1 counts the scale marks, thereby controlling the position of the carriage 1. That the linear encoder scale 7 is positioned in the vicinity of the carriage 1 is preferable for precise control of the position of the carriage 1.
However, disposing the linear encoder scale 7 near the carriage 1 causes a problem in that a user contacts the encoder scale inadvertently and possibly damages it, sheet powder from the paper medium attaches to the encoder scale to smear it, or ink mist as a by-product of the ink droplets is generated and the fine ink droplets coat the surface of the linear encoder scale. If the linear encoder scale 7 is damaged or smeared, the encoder sensor 8 cannot correctly count the scale marks and the position of the carriage 1 cannot be controlled correctly. Accordingly, the linear encoder scale has been designed to be replaceable.
The carriage 1 as illustrated in FIG. 9 reads the scale marks drawn on the linear encoder scale 7 with use of the encoder sensor 8. The encoder sensor 8 includes a light emitter and a light receiver disposed opposite each other with a certain interval therebetween. By passing through the linear encode scale 7 between the light emitter and the light receiver, the encoder scale 8 reads the scale marks. Specifically, the encode sensor 8 has a concave-shaped cross-section if seen from the carriage moving direction, with one open end. The concave-shaped portion includes a slit 81, through which the linear encoder scale 7 is passed. Accordingly, because one end of the slit 81 of the encoder sensor 8 is open, when the user erroneously contacts the linear encoder scale 7, the linear encoder scale 7 detaches from the encoder sensor 8 through an opening of the slit 81. If the linear encoder scale 7 detaches from the encoder sensor 8, the linear encoder scale 7 cannot be read by the encoder sensor 8 and the position of the carriage 1 cannot be controlled.
As a countermeasure, a hole can be provided in a side surface of the carriage for preventing the linear encoder scale from detaching. The linear encoder scale is passed through both the detachment prevention hole and the slit of the encoder scale. With this structure, because the linear encoder scale is supported by an inner wall of the detachment prevention hole in the side surface of the carriage, the linear encoder scale does not easily detach from the encoder sensor.
However, because the linear encoder scale needs to be laterally inserted into the detachment prevention hole on the carriage side surface, the detachment prevention hole cannot be easily observed by human eyes and mounting the long encoder scale to the detachment prevention hole is difficult.